This invention relates to semiconductor chip packaging and, particularly, to stacked package modules.
One approach to increasing the density of semiconductor functionality in semiconductor chip packages is to stack die, one over another, on a package substrate. Electrical interconnection of the stacked die may be die-to-die and/or die-to-substrate, for example by wire bonding. It is technically difficult and therefore costly to test die prior to assembly in the stacked die package, and some number of “bad” die are expected in the supply. One bad die in a stacked die package can render the entire package bad and, accordingly, significant waste of materials and manufacturing time can inevitably result in the manufacture of stacked die packages, particularly where a larger number of die are stacked in the package. Stacked die packages having as many as four, five or six die are in use. Stacked die packages can be made very thin, even where spacers between adjacent die are required.
Another approach to increasing the density of semiconductor functionality in semiconductor chip packages is to stack packages, one over the other, to form a stacked package module. Each package in the module includes at least one die, affixed to and electrically interconnected with, a substrate. The packages in the stack are electrically interconnected, for example by wire bonds between bond sites on the respective package substrates. This has the advantage that the packages can be fully tested before they are assembled in the stack; “bad” packages are discarded, and only packages testing as “good” are used in the module.
Several elements contribute to the thickness of a typical stacked package module. At a minimum, the thicknesses of the respective substrates and die, and die attach adhesive, contribute. Additionally, where the die in each package are molded (or encapsulated) the mold cap has a thickness typically greater than that of the die, and where the die is wire bonded to the substrate the mold cap must be thick enough to completely cover the wires, plus a tolerance for variation in wire loop height. And typically the package stack is also molded or encapsulated with a module molding, which may add additional thickness.
In products where a limit is imposed on both package footprint and package thickness, such as for example in mobile devices such as cellular telephones, PDAs, digital cameras, and various consumer electronic devices, it is desirable to make packages (or modules) as thin as possible without increasing the footprint beyond the limits.